Keying filter



Dec. 8, 1942. c. w. HANSELL.

KEYING FILTER Filed July 21, 1939 m 3m M N ww L m a 0 N Li 0 M ni m \I 8RECTIFIER ww m F m l a: U 8 NEW L2 in LH fin? Q q M 4 a a a m INVEN TOR.an? m HANSELL TIME A TTORN E Y.

-. Patented Dec. 1942 KEYING FILTER Clarence W. lianseil, PortJeflerson, N. Y., asaignor to Radio Corporation of America, acorporation of Delaware Application July 21, 1939, Serial No. 285,691

10 Claims.

This invention relates to energy supply circuits for amplifiers andespecially to a new and useful method and means for supplying anodepotential to a high power amplifier.

In radio signaling systems it is common to use a rectifier, generator orother power supply source having a considerable fluctuation or ripple inits voltage, for supplying anode potentials to amplifiers. In order tosmooth out the uni-directional anode energy, condensers or filterscontaining condensers are placed across the output circuit of the powersource which have very low lmpedances for the ripple energy and so causecurrents to flow through the internal impedance of the rectifier orgenerator of such value that the ripple voltage is substantiallyeliminated from the output.

The power sources used in practice also have a considerable amount ofvoltage regulation. In other words, their output voltages decrease whenthe load current increases. The presence of the smoothing condensersslows up the change of voltagewhen the load current-is changed. In atelegraph transmitter, where the load current fluctuates between zeroand full load in accordance, withlthe keying, the voltage at thebeginning of each dotor dash is higher than the final value by theamount of the} regulation. Consequently, when keyed'energy is firstapplied to the. input side of the amplifier there is often as much as a25% overload voltage existing in the amplifier relative-to the steadymarking voltage.

' Becauseof this, the peak electrical stresses on the amplifier tubesand circuits are g'r'eatlyim creased, tending to causemore.rapiddeteriora "tion, and the total power output rating of theamplifiermust be. reduced to provide the extra safety factor demanded by the.difference between the voltage and the. steady load also=a .markedtendency for the vacuum tubestodevelop parasitic oscillations duringtheoverv'oltage period, which not only increase the voltage and loadstresses'still more,

but which also cause interference in tll 81"" lfld 10 circuits.

.It was an object of my invention described- I in my' United StatesPatent #1-,92 1,858,. dated August 3, 1933, to overcome the foregoingdifliculty and this I- did by reducingthe anode. po-- tential applied totheamplifierwhen energy is" first introduced in the input circuitthereof.

.More specifically, I reduced this potential by inserting in the. leadbetween the anode of the amplifier and the'rectifier an impedance,prefer- I ably a. resistance, which, assoon as signal energy is suppliedto the amplifier, reduces to apredetermined value, the anode potential.For best operation I preferred to make this impedance of a value suchthat when there is a tendency for momentary overloads, under theconditions outlined above, plate voltage is reduced to a value belowthatused for steady marking or steady key down position.

In order to increase the anode voltage during the remaining portion ofthe marking period, I shunted the resistance by a. reactance, in theform of an inductance having low resistance which controlled the rate oftransfer of current from the resistance to itself and hence the voltageto the amplifier anode. However, when energy was removed from theamplifier input, the inductance tended to increase anode potential.

An increase at this time is less harmful than at the beginning of asignal because the radiofrequency voltages are removed, but, it isundesirable, and in some cases can result in sufiicient increase involtage to be destructive. This presents a disadvantage in that, byvirtue of the added potential, stresses in the tubes and circuits areincreased. Therefore, the overcoming of this defect was a further'objectof my patented invention and I did this by resistively dissipating theinductive potential surge which follows the removal of the load on thepower source and which results from the removal of the energy from theinput circuit of the amplifier. Consequently, the impedance formed ofthe resistance and inductance prevents momentary overloads on theamplifier tubes when signaling energy is first introduced therein andprevents excessive plate voltages when signaling energy is removed platevoltage caused by the cyclic charge and discharge of the smoothingcondensers, connected across the rectifier output circuit, through theinductance of the power source and filter circuits.

In'other words, it damps out resonance effects in the power circuitwhich may cause large voltage fluctuations at certain keying speeds ormod-,

ulating'frequen'cies.

The filters serve to reduce the voltage on transmitter tubes at thebeginning of marking periods but increase the supply voltage just afterthe end of each marking period. The transient increase in voltage aftermarking periods tends to make the tubes draw anode currentmomentarilywhen they should not and after the frequency guiding eifectof the radio-frequency input is removed. Under these circumstances,unless the circuits are correctly adjusted, parasitic oscillationssometimes take place during the transient period of high voltage, thusproducing spurious radiations at incorrect frequencies. Such spuriousradiations are known as key click parasitics. 1 v

To reduce the transient voltage increases after marking periods inaccordance with the present invention, I shunt the keying filter with arectifier. This rectifier is so connected and poled that little or nocurrent flows through it while the load current through the keyingfilter is increasing. Now, when the load current is decreasing, currentwill fiow through the rectifier to reduce the voltage surge due to thekeying filter inductance.

My invention is defined with particularity in the appended claims;however, it may best be understood both as to its method of operationand structural organization by referring to the accompanying drawing, inwhich Fig. 1 illustrates a transmitting system incorporating thefeatures of my invention, and

Figs. 2 to 7 inclusive are given by way of explanation thereof.

Referring to Fig. 1, relatively low frequency alternating currentsupplied by source 2 is keyed or otherwise modulated at some centrallylocated point 4, and transmitted over land line 6 to radio transmittingapparatus.

The received keyed energy is rectified in 8. rectifier 8 the output ofwhich is used to control the output of a high frequency oscillator III,for example, of the crystal controlled variety, or of one of theradio-frequency amplifiers or frequency multipliers between a crystalcontrolled amplifier and the power amplifier. The keyed output of theoscillator or amplifier may then be fed into suitable frequencymultipliers and amplifiers l2 which, in turn, supply their energy to thetuned input circuit l4 of a power amplifier l6 having a tuned outputcircuit l8, anodes 20, control electrodes 22 and cathodes 24. Output ofthe power amplifier I6 is fed from its output circuit IE, to, forexample, an antenna 26 and transmitted to a receiving station (notshown).

Anode potential for the power amplifier, is derived from a source ofalternating current 28 coupled through transformer 30'to rectifier 32.For smoothing purposes condensers are connected across the outputcircuit of the rectifier and, for the sake of simplicity, they have allbeen represented as a dotted capacity 34. The lead 36 between therectifier and the anodes of the power amplifier supplies the anodes ofthe amplifier with positive potential.

Because of the inherent regulation of the rectifier, in the absence ofkeyed energy in the input circuit l4 of high power amplifier It, thevoltage applied to the anodes of the amplifier is given by the value 36'of Fig. 2. During steady key down position, the anode potential isrepresented by value 38. Now, at the instant when keyed energy is firstintroduced into the power amplifier, capacity 34 tends to maintain highvoltage on the anodes resulting in the amplifier current input andoutput overload peak 40 of Fig. 3 which indicates the excessive platecurrent flowing at that time. Shortly thereafter, this current drops toits normal value 42, but during the short time that current fiows as atpoint 40 of Fig. 3, the tubes and circuits are subjected to is this poorfrom the apparatus point of view but from the aspect of signaling theincreased current introduces at point 42, Fig. 4, an increased antennacurrent giving rise to undesirable energy modulation which appears as aclick in the receivers.

To overcome the foregoing difficulty in accordance with my inventiondisclosed in the aforesaid patent. I inserted-in series with the anodelead 35 a filter or impedance composed of inductance 44 and resistancewhich'reduced the plate voltage to a value 46 (Fig. 5) much below thenormal plate voltage 38 representing the normal marking plate potential.In this manner, plate current fiow was as shown in Fig. 6, and antennacurrent as indicated in Fig. 7, free from peaks and their attendantevils.

The value of resistance 50 was so adjusted that, at the instant the loadwas thrown on, the voltage drop in the resistance would give the desiredmaximum reduction of voltage on the anodes of the amplifier. It is to benoted that, at the first instant, no current flows in the inductance 44because a potential must be applied for a finite time to build up acurrent in an inductance.

Having arrived at the desired value of resistance 50 the inductance 44is adjusted to give the desired rate of increase on the anodes of theamplifiers from the minimum value up to the normal steady load value. Ifit were desired to have the signal as nearly square as possible then thetime constant of the inductance 44 and resistance 50 could be madesubstantially the same as the time constant of discharge from thesmoothing condenser under the conditions imposed. In practice some ofthe radio-frequency energy output from the amplifier could be rectifiedand observed with an oscillograph. The values of inductance 44 andresistance 50 were then adjusted for the desired signal shape. Usuallythe building up of the signal was slowed down as far as possibleconsistent with the highabnormally high stresses and loads. Not only estsignaling speed to be used. After the values were determined for onetransmitter they could be simply duplicated for similar transmitters.The resistance offered an additional advantage in that if there were anytendency for smoothing reactors or internal inductance of the rectifiertransformers to introduce surges of potential on plates 30 of amplifierHi, the energy cansing the surges would be at least partly dissipated byit. As previously explained, the resistance also absorbs the surge ininductance 44 when the load current is interrupted.

The filter comprising components 44 and 50 serves as described above toreduce the voltage on the tubes at the beginning of the markingconditioninitiating current flow in the antenna smoothly, but due totheir reactance they increase the supply voltage when the markingcondition is. terminated. This transient increase of voltage indicatedat 42", Figur 5, tends to make the tubes draw anode current momentarilywhen they should cut off and after the frequency guiding eifect of theradio-frequency supply to their input is removed. This may causespurious radiations on incorrect frequencies to take place, a form ofspurious radiation sometimes known as key click parasitics." To preventthe reactance of the circuits including 44 from increasingexcessivelythe voltage of marking value after the marking operation hasended, I now shunt the inductance 44 and resistance 50 by a rectifier60. The rectifier as poled passes little or no current during the timethe transmitter is in marking condition and while the current through Nand 50 is increasing. However, when the load current is decreasing,current fiows through the rectifier 60 to thereby reduce the voltagesurge due to the inductive reactance of the filter.

In practice it may be undesirable for the rectifier 60 to have near zeroresistance when it is conducting curent because then the energy in thereactor may not be used up before marking starts again and the filterwould not be fully efi'ective in reducing the transient overloads in themanner intended when it is used. Therefore, I propose to insert someresistance 6| in series with the rectifier to cause the energy in theinductance of the filter to be dissipated more rapidly at the end ofmarking periods.

The irregularity in the voltage supply at 42" is thus smoothed outrelieving overvoltage' stresses and at the end of marking the antennacurrent falls smoothly to zero or its minimum value, as indicated at42", Figure 7.

Any form of rectifier may be employed at 60, provided it has sufiicientability to pass current in the desired direction and can withstand thepotentials developed across it. High vacuum, mercury vapor, neon andother such types of rectifiers known in the art may be used. Copperoxide and similar barrier layer rectifiers might be used, particularlyat the lower voltages.

The resistance 6| in series with the rectifier may be made of Thyrite orsimilar materials having decreasing resistance with increasing potentialacross them and will reduce surge voltage across the reactor 44.

What is claimed is:

1. Means for absorbing energy stored in a reactor, through which passesan interrupted direct current, comprising a rectifier connected acrossthe reactor and a non-linear passing resistor connected in seriesbetween said rectifier and said reactor.

2. Means for absorbing energy stored in, a reactor, through which ispassed an interrupted direct current, comprising a rectifier and aThyrite" resistance in series connected across the reactor.

3. Means for absorbing energy stored in a reactor, through which ispassed a direct current which is then interrupted, comprising arectifier and an automatically variable resistance in series with saidrectifier of material having decreasing resistance with increasingpotential, said rectifier and said variable resistance being connectedin shunt with the reactor.

4. Means for reducing the increase of current into a frequently appliedand interrupted electrical load comprising a resistance and a reactancein parallel connected in series with the load, said rectifier-resistancecombination being connected in parallel, and means for reducing theincrease in potential when the load is interrupted comprising arectifier, with series resistance of material having decreasingresistance with increasing potential, connected in parallel with theresistance and reactance.

5. In a power amplifier, an electron discharge device having an anode, acathode and a control electrode, a high frequency circuit connectedbetween said control electrode and cathode, a high frequency circuitconnected to said anode, a source of anode potential, and, the parallelcombination of an inductance coil, and a circuit including a rectifierand a series resistance of material having decreasing resistance withincreasing potential, said parallel combination being connected betweensaid high frequency anode circuit and said source of potential, saidparallel combination of coil and resistance acting to reduce overloadson said electron discharge device when energy is first introduced intosaid input circuit, by causing the anode current to fiow through saidresistance and subsequently transferring the anode current from saidresistance to said inductance, which inductance in turn is of lowresistance and determines the rate of transfer 01 current from saidresistance to said inductance and said rectifier serving to pass currentin a direction to reduce overloads on said device, on the termination ofenergy in said input circuit, due to the reactance of said parallelcombination.

6. In a signaling system, a power amplifier comprising a pair ofpush-pull connected electron discharge devices, an input circuit withmeans for exciting the same by wave energy, a parallel tuned circuitconnected to the anodes of said electron discharge devices, a source ofanode potential, and a connectioncoupling said source to said paralleltuned circuit, said connection including three parallel branches,namely, an inductance coil, a resistance, and a series circuit includinga rectifier and a non-linear resistance, said parallel branches actingto reduce overloads in said parallel connected pushpull electrondischargedevices, the first mentioned of said resistances acting toreduce supply anode potential to said devices when energy is firstintroduced into said input circuit, the inductance being of lowresistance acting to subsequently supply anode current to said anodesand controlling the rate of transfer of current from said resistance tothe inductance, the rectifier serving to reduce parasitics by passingcurrent in a direction to reduce the supply anode potential whenexcitation is removed from said input circuit.

'7. In combination, a power amplifier having a high frequency anodecircuit and an input circuit, a source of rectified alternating currentfor supplying the anode with unidirectional potential, means forsupplying keyed input energy to the input circuit of the amplifier, acircuit, comprising the parallel connection of an inductance and aresistance, in series with the anode supply lead from the rectifier andbetween the anode circuit and rectifier for reducing anode potentialwhen keyed energy is first introduced into the input circuit of saidamplifier, and means includin a non-linear passive resistance and arectifier for reducing the tendency of the anode potential to increaseon the termination of keyed energy in said input circuit due to thereactance of said parallel connection.

8. In combination, a power amplifier having a high frequency anodecircuit and an input circuit, a source of rectified alternating currentfor supplying the anode with uni-directional potential, means forsupplying keyed input energ to the input circuit of the amplifier, acircuit com prising the parallel connection of an inductance and aresistance, in series with the anode supply lead from the rectifier andbetween the anode circuit and rectifier for reducing anode potentialwhen keyed energy is first introduced into the input circuit of saidamplifier, and means including a resistance and a rectifier for reducingthe tendency of the anode potential to increase on the termination ofkeyed energy in said input circuit due to the reactance of said parallelconnection whereby parasitic oscillations are substantially reduced.

9. In a telegraph transmitter comprising a high frequency amplifierhaving an anode, a grid and a cathode, apparatus for applying keyed highfrequency energy to the grid and the cathode, a source of direct currentpotential and a high frequency output circuit connected between saidanode and cathode, a circuit for reducing parasitics in said outputcircuit at the end of signal character transmissions comprising areactance coil connected in series with and between said direct currentpotential source and said anode.

- shunt to said coil. and a non-linear a series circuit including arectifier connected in resistance, said rectifier operating to dissipateany increase in anode potential due to the reactance of said coil uponthe termination oi keyed high frequency energy in said input circuit.

10. Apparatus as claimed in claim 9 characterized by the fact that a rceis connected in shunt with said reactance coil,

cERTIFIcA'I Qi CORRECTION. Patent No. 2,501+,16'4. I Y December 8, 1%.2.

CLARENCE w. mm'sm.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as fo11ows: Page'3,first column, line 140, c'laiin-l, for ''passing" read --passive-; andthat the acid Letters Patent should be read with this correction thereinthat the same may conform to the record of the caa in the 'PatentOffice.

Sigmd andsealed this- 9th day of February, A. D. 1915.

Henry Van Arsdale, (Seal) Acting Commissioner 01" Patefits.

, cERTIFIc'Amo comcmon. Patent No. 2,50lh16 Y December 8, 1%2.

- CLARENCE w, HAHSELL.

It is hereby certified that error appears in the printed specificatiorof the above numbered pateht requiring correction as follows {Page 5first column, line 1+0, claim-1, for "passing" read --passive--; andthat the seid Letters Patent should be read with this correction thereinthat the same may conform to the record of the case in the PatentOffice.

Signed andsealed this 9th day of Fobruery, A; D. 1%.}.

Henry Van Arsdale,

(Seal) Acting Commissioner of Peteiits.

